Electroacoustic delay lines, especially those operating at ultrasonic frequencies, have found widespread use in various types of electronic equipments; exemplary equipments include radars, computers, color televison receivers and video recorders. A primary function of the delay line is the storage, or delayed transmission, of either analog or digital information for time durations in the microsecond to millisecond range.
Delay lines fabricated from solid transmission media, such as quartz, have assumed a wide variety of shapes and sizes, perhaps the simplest being a substantially straight rod provided with a piezoelectric transducer at each end. The transducers serve to convert electrical signals to ultrasonic waves at the input and ultrasonic waves to electrical signals at the output. See FIG. 1.
Because the time delay from input to output depends, of course, on the effective distance the ultrasonic wave is required to trave, the physical size of the delay line tends to be proportionate to the time delay required. A delay line such as is shown in FIG. 1 would require a rod approaching an impracticable length in order to effect relatively long delays, as are often required. Be aware that typical transmission media require roughly a 2.5 mm effective signal path in order to provide a one microsecond delay. This drawback has impelled the development of delay line configurations more sophisticated than that of the simple rod shown in FIG. 1.
One approach to a reduction in delay line size is characterized by a transmission medium shaped as a regular or an irregular polygon. In such a configuration the signal path is "folded" through the use of multiple reflections at the facets of the polygon. U.S. Pat. No. 2,672,590, entitled "Delay Line", and U.S. Pat. No. 2,839,731, entitled "Multifacet Ultrasonic Delay Line", both by McSkimer et al., exemplify these delay line configurations.
Even longer delays may be effectuated by "double-deck" delay lines such as shown in FIG. 2. The double-deck delay lines in fact comprise two substantially identical delay lines 21 and 22 in which the ultrasonic wave is coupled from one to the other through the operation of a V-shaped coupling wedge 23.
In the polygonally-configured delay lines alluded to above, the number of wave reflections and, consequently, the total delay provided, are limited by the following factors:
(1) As the number of polygonal facets is increased, the tolerance to which the transmission medium is to be ground becomes more stringent in order to circumvent the cumulative effect of deviations from the nominal angles of incidence and reflection at each of the facets.
(2) As the number of reflection is increased, the supression of "spurious" signals becomes more problematic. The spurious signals may be assumed to be at least fourfold in origin:
(i) Signals caused by impedance mismatch at the input or output transducer and which result in reflections at those transducers. These signals will be present at the output at integer multiples of the desired delay time.
(ii) Signals caused by insufficient directivity of the transducer. To wit: signals that travel paths other than the desired signal path and therefore appear at the output displaced by random time intervals from the desired signal.
(iii) Signals derived from reflections of the desired signal at irregularities such as cracks and surface roughness in the solid medium.
(iv) Signals directly coupled from the input to output transducer.
(3) Reduction in the physical size of the transducer is attended by a reduction an the size of the transducers, and a commensurate degradation in the transducer directivity. Degraded transducer directivity contributes to the occurrence of spurious signals alluded to above.
(4) As the number of reflections is increased, the cumulative scattering of the desired signal because of surface roughness or other irregularities at each reflection is increased.
(5) As the number of reflections is increased, the total volume of the transmission medium devoted to the desired signal path is increased. In a typical solid transmission delay line unused portions of the medium (that is, those portions not required for the propogation of the desired signal) are coated with a damping material in order to suppress spurious signals. Because the area allowed to be coated is diminished, the absorbent effect on spurious signals is likewise diminished.
For example, FIG. 3 depects a delay line as it might be used in a color television receiver in order to insert a 63.943 .+-.0.005 microsecond delay. (This delay, equal to approximately the horizontal line period, is often required as a part of video enhancement techniques such as comb filtering.) The delay line in FIG. 3 is characterized by a total of eight reflections occurring at a four facets. The blackened areas on the surface of the delay line represent areas available for the deposition of a damping material. In a specific embodiment, each of these areas might be approximately 2 millimeters square.
By way of comparison, FIG. 4 depicts a delay line of equivalent duration but characterized by twelve reflections occurring at the four facets. Each of the blackened areas is approximately 0.6 millimeter square, respresenting a surface area less than one-tenth that of the areas in FIG. 3. Significantly, the spurious response level of the unit in FIG. 3 was measured at -40 db with respect to the desired signal while the corresponding measurement performed on the unit of FIG. 4 resulted in a figure of -25 db.
Accordingly this invention is directed to a delay line that achieves the desired delay duration in a physically small device through the incorporation of a low number of reflections. The transmission medium should exhibit the desired suppression of unwanted reflections. Furthermore, it is desired that a device with the above attributes be susceptible of large volume, economical production.